High speed xerographic printer

ABSTRACT

A printing apparatus employing a rotating drum having a surface coating comprising a material which exhibits electron beam induced conductivity characteristics. The drum surface rotates through a vacuum chamber in which an electron beam scanning apparatus is situated. The electron beam surface is uniformly charged by suitable means, and is selectively discharged by the scanning electron beam in accordance with a desired pattern to form a corresponding charge pattern on the electron beam sensitive surface. This charge pattern is developed and transferred to a paper surface by xerographic techniques.

United States Patent [1 1 Slack HIGH SPEED XEROGRAPHIC PRINTER [75]Inventor: William Frederick Slack, Andover,

[73] Assignee: Van Dyk Research Corporation,

Whippany, NJ. 22 Filed: May 22, 1972 [21] Appl. No.: 255,387

[52] US. Cl. 346/74 EB, 355/20, 346/74 CR [51] Int. CL... G03b 27/10,G03g 15/00, H04n 5/80 [58] Field of Search 346/74 EB, 74 ES, 74 P,346/74 CR; 118/33, 33 X, 637, 637 X; 226/47; 355/20, 3

[56] References Cited UNITED STATES PATENTS 2,616,961 11/1952 Groak178/5.2 2,630,484 3/1953 Groak 178/5.2 2,972,660 2/1961 Toulon l78/7.22,982,822 5/1961 Bacon 179/100.2 3,099,710 7/1963 Moller 178/6.63,205,301 9/1965 Etcheverry 178/66 ROLLER DRIVE MOTOR PAPER TAKEUP 64DRIVE 1 June 18, 1974 3,222,678 12/1965 Jones 346/1 3,302,900 2/1967Messenger 3,395,401 7/1968 Silverman 3,495,268 2/1970 Hurst 3,681,7778/1972 Smura 346/74 CR Primary Examiner-Paul J. Henon AssistantExaminer-James D. Thomas [5 7] ABSTRACT 8 Claims, 4 Drawing FiguresROUGHING PUMP DIFFUSION PUMP PAIENIEDJUM 1 81914 SHEET 2 85 REE [87 /5s[59 Loop AMP SUPPLY SUPPLY SENSOR MOTOR CAPSTAN l as L 68,69 REE sfl TBELT VAC. LOOP AMP' SENSOR TAKEUP TAKEUP MOTOR CAPSTAN '1 l 89 90 I 65 il l 93\ TONER AMP DISPENSER DISPENSER SENSOR MOTOR WHEEL REF. 94 95 4992 DATA 96 SOURCE INPUT BUFFER AND CONTROL cmcurr VIDEO E 3.

DEFLECTION CIRCUITRY H I01 f SAFETY INTERLOCK PRESSURE FOCUS RUN OUTSENSOR SERVO SENSOR HIGH SPEED XEROGRAPHIC PRINTER This inventionrelates to xerography, and more particularly to a high speed xerographicprinter employing an electron beam exposure technique.

In the practice of conventional xerography, a xerographic surfacecomprising a layer of photoconductive insulating material affixed to aconductive backing is used to create and support electrostatic images.In the usual method of carrying out the process, the xerographic plateis electrostatically charged uniformly over its surface and then exposedto a light pattern of the image being reproduced to thereby dissipatethe charge in the areas where light strikes the photoconductive layer.The undischarged areas of the layer thus form an electrostatic chargepattern in conformity with the configuration of the original lightpattern.

The latent electrostatic image is then developed with a finely dividedelectroscopically attractable (electroscopic) material such as aresinous powder. The powder is held in image areas by the electrostaticcharge on the photoconductive layer. Where the charge is greatest, thegreatest amount of material is deposited; and where the charge is least,little or no material is deposited. Thus, a visible powder image isproduced in conformity with the light image of the copy beingreproduced. The powder image is subsequently transferred to a sheet ofpaper or other surface and suitably affixed (usually by application ofheat) to thereby form a permanent print of the desired image.

ln order to produce copies by means of conventional xerography, anoriginal document must already be available.

In recent years there has been considerable interest in the possibilityof producing xerographic copies directly from electrical data, i.e.without the necessity of first producing an original document by manualor impact printing techniques.

One such technique involves the use of a cathode ray tube to convert theelectrical data into an optical image on the face of the tube, whichimage is employed to expose the photoconductive surface of thexerographic apparatus. However, the necessity for converting theelectrical data into an optical image results in additional complexityand limits the printing speed obtainable.

Another technique which has been proposed employs a character drum whichrotates at high speed adjacent the photoconductive surface. An array ofpulsed light sources situated within the character drum exposes thephotoconductive surface at the moment that the transparency of a desiredcharacter is situated between a corresponding light source and thephotoconductive surface. This technique, however, is limited by thespeed at which the character drum is rotated and the sensitivity of thephotoconductive surface vis-a-vis the light output capability of thepulsed light sources. In addition, this technique imposes difficultsystem synchronization requirements.

An object of the present invention is to provide a high speedxerographic printer capable of converting electrical data directly intoa printed copy.

As herein described, there is provided a high speed xerographic printercomprising a vacuum chamber and an electron beam sensitive surface, afirst portion of which forms a wall of the vacuum chamber. Means isdisposed within the vacuum chamber for scanning the first portion of theelectron beam sensitive surface with an electron beam to establish acharge pattern on said first surface which corresponds to a desiredimage. Means is provided for moving the aforementioned electron beamsensitive surface portion through the vacuum chamber, so that the firstsurface portion leaves the chamber as another portion of the electronbeam sensitive surface enters the chamber.

Means is provided for contacting the first portion of the electron beamsensitive surface with electroscopic toner particles to form a visibletoner pattern corresponding to the desired image. Suitable means isprovided for disposing a portion of a web adjacent the first I surfaceportion. Web supply drive means is provided for feeding the web towardsthe first surface portion, and web takeup drive means is provided fordrawing the web away from the first surface portion.

Means is also provided for transferring the toner pattern to the webportion, and for fusing the transferred toner pattern to the web.

In the drawing:

FIG. 1 shows a high speed xerographic printer according to a preferredembodiment of the present invention;

FIG. 2 shows a block diagram of certain control circuits employed inconjunction with the printer of FIG.

FlG. 3 shows a block diagram of the circuitry associated with theelectron beam unit employed in the printer of FIG. 1; and

H6. 4 is a cross-sectional view of the anode region of the electron beamunit employed in the printer of FIG. 1, and the adjacent portion of theelectron beam sensitive surface employed in said printer.

The high speed xerographic printer 10 shown in FIG. 1 comprises a numberof operating stations situated about the periphery of a generallycylindrical rotatable drum 11.

The drum 11 is rotatably mounted on an axle 12, and is continuallyrotated (in the direction indicated by the arrow 13) by a capstan l4situated within the drum 11 and engaging the interior surface thereof.The capstan 14 is rotated by a suitable drive motor (not shown), whichmay also be situated within the drum 11.

The drum 11 may be fabricated of a suitable electrically conductivemetal, such as steel or aluminum, the peripheral surface of the drum 11being coated with a relatively hard electron beam sensitive material 15.

By the tem electron beam sensitive material as herein employed, is meanteither (i) an insulating material capable of being charged or dischargedby a scanning electron beam, or (ii) a material which is initiallyinsulating or of high resistivity, and which exhibits electron beaminduced conductivity characteristics such that the conductivity of thematerial may be substantially increased by a scanning electron beam.

Preferably, the latter mentioned type of material is employed for thecoating 15 on the peripheral surface of the drum ll. Suitable materialsare described on pages 33 to 51 of Electronic Image Storage by B. Kazanand M. Knoll, Academic Press, 1968, New York and London. and may bedeposited pyrolytically or by any other suitable technique on theperiphery on the drum 11.

For a typical high speed application where it is desired to print copyat the rate, e. g., of 6 feet per second, the outside diameter of thedrum ll may be 5.7

feet corresponding to a circumference of 18 feet, and the drum may berotated at an angular velocity of 20 rpm. 1

The operation of the high speed printer commences with the charging ofthe electron beam sensitive coating as the drum 11 rotates under acorona emitter or corotron 16, which is operated at a predetermined DCpotential with respect to the conductive drum 11. The corotron 16charges the electron beam sensitive coating 15 to establish a uniformelectrostatic charge density thereon.

As the drum 11 continues to rotate, the charged electron beam sensitivecoating 15 enters the vacuum chamber 17, the portion of the coating 15situated within the vacuum chamber 17 forming a wall of the vacuumchamber.

The vacuum chamber 17 consists of an inner chamber portion 18 which issurrounded by an outer chamber portion 19. The wall portions of theouter chamber portion 19 adjacent the electron beam sensitive coating 15are spaced from the drum l1, and curved adjacent the ends 20 thereof toconform closely to the shape of the sealing rollers 21. The sideportions of the outer chamber portion 19 extend to form flanges along aportion of the sides of the drum 11, and are situated closely adjacentto said drum sides. These side flanges 22 are covered with a suitablesealing material, such as felt, which bears against the rotating sideportions 23 of the drum 11. The sliding joints between the flanges 22 ofthe outer vacuum chamber portion 19 and the adjacent side portions 23 ofthe drum 11 are sealed by a suitable vacuum lubricant. I

A vacuum seal between the end portions 20 of the outer chamber portion19 and the adjacent portions of the electron beam sensitive coating 15on the drum 11 is maintained by the resilient sealing rollers 24, whichare rotated by the coating 15, and which slide against the end portions20 of the side walls of the outer chamber portion 19 of the vacuumchamber 17. The surfaces of the end portions 20 which contact theresilient rollers 24 are fabricated of highly polished resilient metal,but are sufficiently rigid so as to elastically deform the adjacentportions of the rollers 24 to maintain a tight seal therebetween.

The sealing rollers 24 may preferably comprise an outer sheath of a lowsurface friction material such as polytetrafluoroethylene, an underlyingresilient layer of silicone rubber, and an inner core of a rigid metalsuch as steel. These rollers may be of the general type shown in U.S.Pat. No. 3,435,500.

The lengths of the rollers 24 are slightly greater than the length ofthe drum 11, the protruding ends of the rollers 24 abutting againstcorresponding portions of the flanges 22 to form a vacuum seal at thesides 23 of the drum 1!.

Similarly, the end portions 25 of the side walls of the inner vacuumchamber 18 contact the inner resilient sealing rollers 26 to form avacuum seal therebetween, the rollers 26 being rotated by contact withthe electron beam sensitive coating 15 on the periphery of the drum 11,the rollers 26 acting to provide a vacuum seal between the coating 15and the side walls of the inner vacuum chamber portion 18. The sealingrollers 26 have lengths slightly greater than that of the drum 11, theflanges 27 at the ends of the side walls of the inner chamber portion 18abutting against the protruding portions of the sealing rollers 26 andthe adjacent side portions 23 of the drum 11 to form a vacuum sealtherebetween. The sealing rollers 26 may be of the same generalconstruction as the sealing rollers 21.

With the aforementioned arrangement, a portion of the electron beamsensitive coating 15 forms a wall of the vacuum chamber 17, i.e. of theinner vacuum chamber portion 18 as well as the outer vacuum chamberportion 19.

A high capacity roughing pump 28 continuously evacuates the outer vacuumchamber portion 19 to create and maintain a vacuum therein. The innervacuum chamber portion 18 of the vacuum chamber 17 is continuallyevacuated by a diffusion pump 29, which is of lower volumetric capacitythan the roughing pump 28, but capable of reducing the pressure withinthe compartment being evacuated to a lower value. The diffusion pump 29is selected so as to maintain a pressure within the inner vacuum chamberportion 18 on the order of 10 torr or less.

A dust shield 30 extends beyond the flanges 22 and 27, and protects themfrom contamination by airborne toner particles, dust, etc., whichcontaminants may adversely affect the vacuum seal between said flangesand the adjacent portions of the sealing rollers and the sides 23 of thedrum l1.

Situated within the inner vacuum chamber portion 18 is an electron beamunit 31, which is secured to the inner walls of the chamber portion 18by a number of vibration isolation mounts 32. The electron beam unit 31comprises a glass supporting envelope 33 having an open end portionadjacent the electron beam sensitive coating 15.

Situated within the supporting envelope 33 of the electron beam unit 31are an electron gun 34, electrostatic deflection plates 35, and anaccelerating electrode 37. A magnetic focus coil 38 surrounds a portionof the neck of the envelope 33 of the electron beam unit 31.

The accelerating electrode 37 may comprise a metallic electrode situatedwithin the glass envelope 33, or alternatively may be in the form of aconductive coating on the inner surface of the envelope 33 adjacent theopen end thereof.

The open end of the glass envelope 33 terminates in a flange 39, thesurface of the flange 39 adjacent the electron beam sensitive coating 15being covered with a conductive coating 40 which may be utilized as asensor to measure the distance between the envelope 33 and the coating15 by monitoring the capacitance between the conductive coating 40 andthe adjacent portion of the conductive drum 11.

By application thereto of suitable operating potentials, the electronbeam unit 31 is caused to generate and scan an electron beam indicatedby the dashed line 41, which beam is caused to impinge upon the adjacentportion of the electron beam sensitive coating 15 which forms a wall ofthe inner vacuum chamber portion 18. Those portions of the coating 15which are exposed to the scanning electron beam 41 exhibit a substantialincrease in conductivity, and are thereby discharged, i.e. lose thecharge which has been placed thereon by the corotron 16. Therefore, bydeflecting the scanning electron beam 41 in a suitable pattern, anelectrostatic charge pattern is established on the electron beamsensitive coating 15, which charge pattern corresponds to a desiredimage.

As the drum ll continues to rotate, the portion of the electron beamsensitive coating containing the electrostatic charge pattern defined bythe scanning electron beam 41 leaves the vacuum chamber 17, as an otherportion of the coating 15 enters said chamber.

The latent electrostatic image on the electron beam sensitive surface 15enters the developer tank 42, whose function is to convert the latentelectrostatic image to a visible toner pattern.

Situated in the lower portion or sump region of the developer tank 42 isa granular developer mixture 43, which consists of resin coated steelbeads and electroscopic toner powder. The developer mixture 43 iscontinually carried from the sump region of the developer tank 42 to ahopper 44 at or near the top of the developer tank, by means of aconveyor belt 45 having a plurality of carrier buckets 46 affixedthereto.

As the drum 11 rotates to move the portion of the electron beamsensitive coating 15 having the desired latent electrostatic imagethereon through the developer tank 42, the developer mixture in thehopper 44 continually flows out the orifice at the bottom of the hopper,and cascades over the electron beam sensitive surface 15 of the drum 11.As the developer mixture cascades over the electron beam sensitivesurface, toner particles are attracted away from the moving carrierbeads and adhere to the discharged portions of the electron beamsensitive surface, thus converting the latent electrostatic imagethereon to a corresponding visible pattern of pigmented or dyed tonerparticles. This technique is commonly referred to as reversaldevelopment. The developer mixture (less any toner particles which haveadhered to the latent electrostatic image on the electron beam sensitivesurface 15 of the drum 1 1) falls back into the sump of the developertank 42 after the developer has cascaded over the electron beamsensitive surface 15.

The direction of rotation of the drum 11 is such that the latentelectrostatic image on the electron beam sensitive surface 15 continuesto encounter fresh" toner particles as the electrostatic charge patternmoves upward through the developer tank 42. This reverse" cascadedevelopment technique provides improved uniformity of image developmentover the conventional forward development technique, wherein the latentelectrostatic image moves through the developer tank in the samedirection as that in which developer mixture is cascaded over thepattern to be developed.

The proper concentration of toner in the developer mixture 43 situatedin the sump of the developer tank 42 is maintained by a toner dispenserunit 47, which contains a supply of toner powder 48 and dispenses thetoner powder to the sump of the developer tank 42 by means of adispenser wheel 49, which when rotated causes toner from the reservoir48 to be transferred to the sump of the developer tank 42 through anorifice 50.

The operation of the dispenser wheel 49 is controlled by a suitabletoner concentration control circuit, which continually monitors theconcentration of toner in the sump of the developer tank, and operatesthe dispenser wheel 49 in such a manner as to maintain the tonerconcentration in the developer tank sump at a desired predeterminedvalue. The toner concentration control circuit may be of the generaltype described in US. patent application Ser. No. 227,965, filed Feb.22, 1972 and assigned to the assignee of the instant application.

Upon further rotation of the drum 11, the visible pattern of tonerparticles (toner particles being electroscopically adherent todischarged areas of the latent electrostatic image on the electron beamsensitive surface 15) is brought into juxtaposition with a moving paperweb 51, which is attracted to the electron beam sensitive coating 15 byelectrostatic induction forces, and thereby caused to progress at aspeed equal to the peripheral speed of the drum 11, so that there issubstantially no relative motion between the paper web 51 and theadjacent portion of the periphery of the drum 11.

In order to isolate the portion of the paper web 51 in contact with thedrum 11 from the remaining portions of the paper web 51, thus insuringthat the speed of the portion of the paper web 51 in contact with thedrum 11 is determined by the peripheral speed of the drum 1 l, in orderto minimize any possibility of slippage with resultant blurring or otherdistortion of the toner pattern, two slack loops 52 and 53 are formed inthe web 51.

By referring to the loops 52 and 53 in the web 51 as being slack" loops,it is not meant that there is no tension whatsoever in the portion ofthe web 51 within these loops. Rather, it is meant only that the tensionof the portion of the web within these loops is substantially less thanthe tension in the portion of the web extending on one or the other sideof each loop. Thus, in FIG. 1 the tension in the portion of the paperweb 51 to the right of the slack loop 52 is substantially greater thanthe tension of the portion of the web in the loop itself, while thetension in the portion of the web 51 to the left of the slack loop 53 issubstantially greater than the tension of portion of the web within thatloop.

The slack loop 52 is situated within and established and maintained by avacuum chamber 54, which has a lower orifice 55 connected to a suitablevacuum source (not shown), In order to sense the size of the slack loop52, a light source 56 and a photodetector 57 are provided on oppositesides of the vacuum chamber 54, which is provided with transparent wallportions in the path of the light emanating from the source 56 toilluminate the detector 57.

As the size of the loop 52 increases, the bottom portion of the loopintercepts all or part of the light reaching the photodetector 57, thuscausing the photodetector to provide an output signal indicative of thesize of the loop 52. The output of the photodetector 57 is coupled to asuitable circuit to control the paper supply drive motor 58, which ismechanically coupled to the paper supply drive capstan 59.

The capstan 59 cooperates with the pinch roller 60 to drive the paperweb 51 toward the slack loop 52 at a rate so as to maintain the slackloop 52 at a size such that the bottom of the loop just intercepts thelight beam extending between the light source 56 and the photodetector57. if desired, a more responsive and smoother operating control systemcan be realized by providing a number of light sources andphotodetectors in vertically aligned relationship, with the outputs ofthe photodetectors coupled to a suitable logic circuit to provideproportional control of the paper supply drive motor 58, rather than themotor operation obtainable with a single photodetector unit.

Similarly, the slack loop 53 is situated within an inverted vacuumchamber 61, which has a lower orifice 62 therein coupled to a suitablevacuum source (not shown).

The size of the inverted loop 53 is sensed by a photodetector 63, whichis illuminated by a light source 64 through aligned transparentapertures in the walls of the vacuum chamber 61.

The output of the photodetector 63 is coupled to a suitable controlcircuit, which operates the paper takeup drive motor 65. The papertakeup drive motor 65 is mechanically connected to the paper takeupdrive capstan 66, which cooperates with the pinch roller 67 to drive theexiting portion of the paper web 51 at a speed such that the bottom ofthe slack loop 53 just intercepts the beam extending between lightsource 64 and photodetector 63.

The paper takeup drive motor 65 is also mechanically coupled to therollers 68 which drive the fuser belt 69 In order to further minimizeany external drag on the portion of the paper web 51 in contact with thedrum 11, the idler rollers 70, 71 and 72 situated along the path of theweb 51 between the slack loops 52 and 53, are driven at a peripheralspeed just slightly less than that of the drum 11. The rollers 70, 71and 72 are driven through overrunning clutches 73, 74 and 75perspectively, by the roller drive motor 76. Both the roller drive motor76 and the motor which drives the drum drive capstan 14 and thereforethe drum 11, are of the synchronous type (i.e. their speed is determinedby the power supply frequency and is substantially unaffected by loadvariations), to insure accuracy of rotational speed. The roller drivemotor 76 operates to drive the idler rollers 70, 71 and 72 at aperipheral speed equal to approximately 98 percent of the peripheralspeed of the drum 11. This action insures that there is very littlerelative movement between the portion of the paper web 51 in proximityto the drum 11, and the idler rollers 70, 71 and 72. The overrunningclutches 73, 74 and 75 permit the idler rollers to which they arerespectively connected to become uncoupled from the roller drive motor76 in the event that the speed of one or more idler rollers is increasedby frictional engagement with the web 51.

An additional idler roller 77 is situated at the left of the invertedloop 53 in order to guide the web 51 into the fuser oven 78.

As the toner pattern (corresponding to the desired image as scanned bythe electron beam 41 of the electron beam unit 31) passes the movingpaper web 51 in close proximity thereto, an image transfer coronaemitter or corotron 79 attracts the toner particles away from theelectron beam surface and onto the adjacent portion of the moving paperweb 51. The toner pattern is thus transferred onto the moving paper web51.

The moving paper web 51, having the pattern of toner particles adherentthereto by electrostatic attraction forces, then moves around the idlerroller 72, through the slack loop 53 within the vacuum chamber 61, andaround the idler roller 77 into the fuser oven 78. A brake 110 incontact with the idler roller 77 maintains proper tension in the portionof the web 51 to the left of the slack loop 53.

The purpose of providing an inverted loop 53 is to insure that the idlerrollers 72 and 77, as well as the walls of the vacuum chamber 61,contact only the back surface of the paper web 51, i.e. the surfaceopposite that on which the toner pattern is disposed. This arrangementinsures that the toner pattern will not be smeared or otherwisedistorted before it is fused to the paper web 51.

The paper takeup drive motor 65 draws the paper web 51 from the slackloop 53 through the fuser oven 78, which is maintained at a temperaturesufficiently high to fuse the toner pattern to the paper surface, whileat the same time being sufficiently low so that the paper web 51 is notscorched or otherwise deteriorated.

The fuser oven 78 is of elongated configuration, preferably having alength on the order of 6 feet for a paper web speed of 6 feet persecond.

The paper takeup drive motor 65 also rotates a plurality of fuser beltdrive rollers 68, which drive the fuser belt 69 at a speed equal to thatof the paper web 51. The fuser belt 69 comprises a porous material, theinterior region encompassed by the fuser belt being connected to asuitable vacuum source (not shown), so that the paper web 51 is retainedin contact with the fuser belt 69 by differential air pressure due tothe action of the vacuum source.

The fuser belt 69 is situated above the paper web 51, so that the beltcontacts the back surface of the web and does not touch the tonerpattern being fused thereto.

As the drum l1 continues to rotate, it passes beneath the alternatingcurrent corona emitting device or corotron 80, which neutralizes anyresidual charge remaining on the electron beam sensitive surface 15,thus reducing or cancelling the electrostatic attraction between anyresidual toner particles and the adjacent electron beam sensitivesurface.

A rotating brush 81 situated in a substantially dusttight compartment 82mechanically removes any remaining toner particles from the electronbeam sensitive surface 15 of the drum 11, the toner particles so removedbeing drawn out through the conduit 83 by air pressure as a result ofthe application of a suitable vacuum source (not shown) to the conduit83. Before the toner-laden air is returned to the atmosphere, it isfiltered by suitable means (not shown) to remove the toner particlestherefrom.

After the residual toner particles have been removed from the electronbeam sensitive surface 15 by the action of the brush 81 and the vacuumsource associated therewith, the electron beam sensitive surface 15passes beneath an alternating current corona emitting device or corotron84, which substantially reduces any residual charge remaining on thesurface 15, and substantially reduces any static effects introduces bytriboelectric interaction between the rotating brush 81 and the electronbeam sensitive surface 15.

Thereafter, the corona emitting device or corotron 16 recharges theelectron beam sensitive surface 15 in a uniform manner, in preparationfor the next cycle of machine operation.

The manner of operation of the vacuum loop control systems and the tonerconcentration circuitry is illustrated in block diagram form in FIG. 2.

The output of the supply loop sensor 57 is compared to a signalindicative of a desired loop size from a reference circuit 85, in acomparator 86, the resulting correction signal being applied to anamplifier 87, the output of which drives the supply motor 58, which ismechanically coupled to the supply capstan 59. The resulting servo loopmaintains the size of the slack loop 52 at a value corresponding to thatindicated by the reference circuit 85.

In similar fashion, the output of the takeup loop sensor 63 is comparedto a signal indicative of a desired loop size, as generated by thereference circuit 88, in a comparator 89, the resulting correctionsignal being applied to an amplifier 90, the output of which drives (i)the fuser drive rollers 68 which in turn drive the fuser support belt69, and (ii) the paper takeup drive motor 65, which in turn is coupledto the takeup capstan 66.

The concentration of toner within the developer mixture 43 in the sumpof the developer tank 42 is monitored by a toner sensor 91 (not shown inFIG. 1), the output of which is compared to a signal generated by thereference circuit 92 and indicative of a desired toner concentration.The comparison is made in a comparator 93, and the resulting correctionsignal is applied to the amplifier 94, the output of which drives thedispenser motor 95, which in turn is mechanically coupled to thedispenser wheel 49 shown in FIG. 1.

By this arrangement, the dispenser wheel 49 is operated in such a manneras to maintain the concentration of toner within the developer mixture43 in the sump of the developer tank 42 at a constant predeterminedvalue corresponding to the setting of the reference circuit 92.

The circuitry provided for causing the electron beam unit 31 to scan adesired pattern so as to form a corresponding latent electrostatic imageon the electron beam sensitive surface 15, is shown in FIG. 3.

The data to be printed originates from a data source 96, which maytypically be a computer or data transmission terminal. This data istransferred to an input buffer and control circuit 97, which convertsthe input data to a line scan format, and provides buffering between thespeed at which information is supplied from the data source 96, and thespeed at which information is to be delivered to the electron beam unit31.

The modified and buffered signals are coupled from the circuit 97 to thevideo and deflection circuitry 98, which provides video and deflectionsignals of proper values and at suitable power levels to drive theelectron beam unit 31, the line 99 denoting the deflection signals tothe deflection plates 35 of the electron beam unit 31, and the line 100denoting the video signals delivered thereto.

A deflection monitoring signal is provided by the video and deflectioncircuitry 98 to the safety interlock circuit 101 on line 102. Thismonitoring signal is a DC level which has one value whenever deflectionsignals are being supplied to the electron beam unit 31, and anothervalue when such signals are absent.

Whenever the deflection signals are absent, the safety interlock circuit101 provides a signal to the electron gun 34 of the electron beam unit31 on line 103, to cut off the electron beam 41, thereby preventingdamage to the electron beam sensitive surface when proper deflectionsignals are not being applied to the electron beam unit 31.

The safety interlock circuit 101 also receives data from a pressuresensor 104, which is situated within the inner vacuum chamber portion 18of the vacuum chamber 17. The safety interlock circuit cuts off theelectron beam 41 whenever the pressure sensor 104 indicates that thereis insufficient vacuum, i.e. excessive pressure within the inner vacuumchamber portion 18.

In order to obtain high imaging resolution, the electron beam 41 isfocused to a small point by the action of the electron gun 34 and thefocus coil 38. As a result, the depth of field of the resulting scannedelectron beam pattern is limited, and small variations in the distancebetween the adjacent surface of the electron beam sensitive coating 15and the electron beam unit 31 may result in loss of resolution.

In order to minimize variation of this distance, the drum drive capstan14 is arranged to contact the inner surface of the drum 1 1 at a pointopposite that at which the electron beam 41 impinges on the surface 15.Thus any runout of the drum 11 is minimized at the point where theelectron beam 41 scans the surface 15.

In order to compensate for any remaining small variations in thedistance between the electron beam sensitive surface 15 and the electronbeam unit 31, a runout sensor 40 and focus servo 105 are provided.

The conductive coating which acts as the runout sensor 40 provides anoutput signal which is a measure of the distance between the conductivedrum 11 and the flange 39 of the glass envelope 33 of the electron beamunit 31 (see FIG. 4), which signal is employed to vary the current tothe focus coil 38 in corresponding fashion. The output of the runoutsensor 40 is supplied to the focus servo 105, which in turn supplies thedrive current for the focus coil 38.

As shown in FIG. 4, the runout sensor 40 comprises a conductive filmdisposed on the outer flange 39 of the glass envelope 33 of the electronbeam unit 31, the coating 40 extending into an aperture 106 in theflange 39. A suitable connector 107 extends into the aperture 106 tomake electrical contact with the coating 40.

In similar fashion, an electrical connector 108 extends into an aperture109 in the glass envelope 33 to make electrical contact with theaccelerating electrode 37.

As the distance between the electron beam unit 31 and the drum 11varies, a corresponding variation occurs in the capacitance between therunout sensor 40 and the drum 11. This capacitance variation is utilizedby the focus servo 105 to correspondingly vary the current to the focuscoil 38, thereby to vary the point of focus of the electron beam 41, sothat said focus point always occurs at the electron beam sensitivesurface 15.

I claim: 1. A high speed xerographic printer, comprising: a vacuumchamber; an electron beam sensitive surface comprising the periphery ofa generally cylindrical drum mounted for rotation about a longitudinalaxis thereof, a first portion of said surface forming a wall of saidchamber; means disposed within said chamber for scanning said surfaceportion with an electron beam to establish a charge pattern thereoncorresponding to a desired image;

means for moving said surface portion through said chamber, so that saidfirst portion leaves said chamber as another portion of said surfaceenters said chamber;

means for contacting said first surface portion with electroscopic tonerparticles to form a visible toner pattern corresponding to said desiredimage;

means for disposing a portion of a web adjacent said first surfaceportion;

web supply drive means for feeding said web toward said first surfaceportion;

web takeup drive means for drawing said web away from said first surfaceportion;

means for producing and maintaining a first slack loop in said webbetween said web portion and said web supply drive means;

means for producing and maintaining a second slack loop in said webbetween said web portion and said web takeup drive means;

means for transferring said toner pattern to said web portion; and

means for fusing said transferred toner pattern to said web.

2. The printer according to claim 1, further comprising first and secondsensors responsive to the sizes of said first and second loopsrespectively, means for controlling said web supply drive means inresponse to the output of said first sensor to maintain the size of saidfirst loop within a first desired range, and means for controlling saidweb takeup drive means in response to the output of said second sensorto maintain the size of said second loop within a second desired range.

3. The printer according to claim 2, further including a runout sensorfor providing a signal responsive to the distance between said scanningmeans and the adjacent portion of said electron beam sensitive surface,and means responsive to said signal to cause said beam to focus on saidadjacent surface portion irrespective of variations in said distance.

4. The printer according to claim 1, further including means forrotating said drum about said axis.

5. The printer according to claim 4, wherein said rotating means drivesthe periphery of said drum in a region adjacent said scanning means.

6. A high speed xerographic printer, comprising:

a vacuum chamber comprising an inner and an outer chamber portion, saidinner chamber portion being sealed with respect to said outer chamberportion;

an electron beam sensitive surface, a first portion of said surfaceforming a wall of said chamber and being spaced from two adjacent wallsof said chamher; two resilient rollers in contact with said firstsurface portion, each roller also contacting a corresponding one of saidadjacent walls in such a manner that said rollers form seals betweensaid first surface portion and said adjacent walls;

means disposed within said inner chamber portion for scanning saidsurface portion with an electron beam to establish a charge patternthereon corresponding to a desired image;

means for moving said surface portion through said chamber, so that saidfirst portion leaves said chamber as another portion of said surfaceenters said chamber; means for contacting said first surface portionwith electroscopic toner particles to form a visible toner patterncorresponding to said desired image;

means for disposing a portion of a web adjacent said first surfaceportion;

web supply drive means for feeding said web toward said first surfaceportion;

web takeup drive means for drawing said web away from said first surfaceportion;

means for transferring said toner pattern to said web portion; and

means for fusing said transferred toner pattern to said web.

7. The printer according to claim 6, wherein said electron beamsensitive surface comprises the periphery of a generally cylindricaldrum mounted for rotation about a longitudinal axis thereof.

8. The printer according to claim 6, further including means fordisabling said scanning means when the pressure within said innerchamber portion exceeds a predetermined value.

1. A high speed xerographic printer, comprising: a vacuum chamber; anelectron beam sensitive surface comprising the periphery of a generallycylindrical drum mounted for rotation about a longitudinal axis thereof,a first portion of said surface forming a wall of said chamber; meansdisposed within said chamber for scanning said surface portion with anelectron beam to establish a charge pattern thereon corresponding to adesired image; means for moving said surface portion through saidchamber, so that said first portion leaves said chamber as anotherportion of said surface enters said chamber; means for contacting saidfirst surface portion with electroscopic toner particles to form avisible toner pattern corresponding to said desired image; means fordisposing a portion of a web adjacent said first surface portion; websupply drive means for feeding said web toward said first surfaceportion; web takeup drive means for drawing said web away from saidfirst surface portion; means for producing and maintaining a first slackloop in said web between said web portion and said web supply drivemeans; means for producing and maintaining a second slack loop in saidweb between said web portion and said web takeup drive means; means fortransferring said toner pattern to said web portion; and means forfusing said transferred toner pattern to said web.
 2. The printeraccording to claim 1, further comprising first and second sensorsresponsive to the sizes of said first and second loops respectively,means for controlling said web supply drive means in response to theoutput of said first sensor to maintain the size of said first loopwithin a fIrst desired range, and means for controlling said web takeupdrive means in response to the output of said second sensor to maintainthe size of said second loop within a second desired range.
 3. Theprinter according to claim 2, further including a runout sensor forproviding a signal responsive to the distance between said scanningmeans and the adjacent portion of said electron beam sensitive surface,and means responsive to said signal to cause said beam to focus on saidadjacent surface portion irrespective of variations in said distance. 4.The printer according to claim 1, further including means for rotatingsaid drum about said axis.
 5. The printer according to claim 4, whereinsaid rotating means drives the periphery of said drum in a regionadjacent said scanning means.
 6. A high speed xerographic printer,comprising: a vacuum chamber comprising an inner and an outer chamberportion, said inner chamber portion being sealed with respect to saidouter chamber portion; an electron beam sensitive surface, a firstportion of said surface forming a wall of said chamber and being spacedfrom two adjacent walls of said chamber; two resilient rollers incontact with said first surface portion, each roller also contacting acorresponding one of said adjacent walls in such a manner that saidrollers form seals between said first surface portion and said adjacentwalls; means disposed within said inner chamber portion for scanningsaid surface portion with an electron beam to establish a charge patternthereon corresponding to a desired image; means for moving said surfaceportion through said chamber, so that said first portion leaves saidchamber as another portion of said surface enters said chamber; meansfor contacting said first surface portion with electroscopic tonerparticles to form a visible toner pattern corresponding to said desiredimage; means for disposing a portion of a web adjacent said firstsurface portion; web supply drive means for feeding said web toward saidfirst surface portion; web takeup drive means for drawing said web awayfrom said first surface portion; means for transferring said tonerpattern to said web portion; and means for fusing said transferred tonerpattern to said web.
 7. The printer according to claim 6, wherein saidelectron beam sensitive surface comprises the periphery of a generallycylindrical drum mounted for rotation about a longitudinal axis thereof.8. The printer according to claim 6, further including means fordisabling said scanning means when the pressure within said innerchamber portion exceeds a predetermined value.